Our invention relates to semiconductor devices capable of supporting high reverse voltages while in a current-blocking state and, more particularly, to such devices having means for reducing the on-resistance (or anode-to-cathode resistance while "on" or conducting) of the devices.
The above-referenced application describes a high voltage P-N diode including a thin N.sup.- (or lightly-doped N-type) layer atop a P.sup.- substrate. An N.sup.+ (or highly-doped N-type) cathode region extends into the N.sup.- epitaxial layer from its upper surface, and a P.sup.+ anode region extends into the N.sup.- epitaxial layer from its upper surface and surrounds the N.sup.+ cathode region. To reduce parasitic currents in the device, an N.sup.+ buried layer exists between the P.sup.- substrate and the N.sup.- epitaxial layer, beneath the P.sup.+ anode region, so as to surround the N.sup.+ cathode region (when viewed from above). In order to avoid the loss of voltage-supporting capability in the N.sup.- epitaxial layer between the N.sup.+ cathode region and the N.sup.+ buried layer, a further P.sup.+ region extends into the N.sup.- epitaxial layer from its upper surface and is interposed between the N.sup.+ cathode region and the P.sup.+ anode region so as to surround the N.sup.+ cathode region and, in turn, be surrounded by the P.sup.+ anode region. The further P.sup.+ region is permanently biased at or near the potential the P.sup.- substrate, whereby the N.sup.- epitaxial layer, between the further P.sup.+ region and the N.sup.+ cathode region, supports reverse voltages in the device. The foregoing P-N diode attains a reasonably low value of on-resistance. It would be desirable, however, to provide a diode having an even lower value of on-resistance.
The above-referenced patent application describes (and claims) a bipolar transistor which is structurally similar to the foregoing P-N diode. The present invention addresses a similar concern about the desirability of reduced on-resistance of a bipolar transistor.